Communication network technology is advancing at a rapid rate. For example, all-optical networks using wavelength division multiplexing (WDM) are currently being deployed for a wide variety of communication applications. WDM techniques allow optical signals having different wavelengths to be multiplexed into a single optical fiber. Each of the wavelengths serves as an optical carrier and can be used independently of the other wavelengths, such that different wavelengths may use different modulation formats to carry different signal types. In one possible implementation, each wavelength may carry a modulation signal representing a synchronous optical network/synchronous digital hierarchy (SONET/SDH) client payload, where each client is a SONET-rate time division multiplexed (TDM) application and the common carried signals are in an OC-x format, where “OC” denotes optical carrier and x denotes the rate, e.g., an OC-3 format, an OC-48 format, an OC-192 format, etc.
Such optical networks generally include routing elements such as wavelength switching cross-connects, wavelength adapters, wavelength interchanging cross-connects, etc. A wavelength switching cross-connect serves to cross-connect incoming wavelengths on a given input fiber to different output fibers, but does not provide any transformation in wavelength. When only this type of routing device is present in an optical network, the network typically routes a given end-to-end demand using a single wavelength. If a primary network path assigned to the given demand fails, the demand generally must be carried on a secondary or restoration path using exactly the same wavelength as the primary path. A wavelength adapter is a device which allows conversion of wavelength at the client-network interface. A wavelength interchanging cross-connect is used to cross-connect incoming wavelengths onto different output fibers while also providing transformation of wavelengths.
One type of approach to providing failure protection in an optical network formed of these and other elements is to provide complete redundancy, such that the network includes a dedicated backup or secondary connection for each primary connection of the network. When a link, span or node of the primary connection fails, traffic may then be switched onto the corresponding elements of the secondary connection.
FIG. 1 shows an example traffic demand between two nodes A and Z of a given network. In this example, the demand is two units of OC-x traffic, each unit corresponding to one of the dashed lines between nodes A and Z.
FIG. 2 illustrates a conventional network protection approach as applied to the two units of OC-x traffic in the FIG. 1 example. This approach is an example of the above-noted complete redundancy approach, and is known in the art as “1+1” protection or “bridge and select.” In this approach, the first and second units of OC-x traffic are routed in the manner indicated at 100 and 110, respectively. More particularly, an original signal corresponding to the first unit of OC-x traffic is routed on a first link 102 between the nodes A and Z, while a copy of this signal is routed on a second link 104 between the nodes A and Z. A particular one of the original signal or the copy is then selected at the destination node Z. The second unit of OC-x traffic is routed in a similar manner, with an original signal corresponding to the second unit of OC-x traffic routed on a link 112 between the nodes A and Z, while a copy of this signal is routed on a link 114 between the nodes A and Z. Again, a particular one of the original signal or the copy is then selected at the destination node Z.
The “bridge and select” approach illustrated in FIG. 2 provides complete redundancy for the capacity required to route the two units of OC-x traffic between nodes A and Z. However, this approach suffers from a number of significant drawbacks. For example, although the approach provides link protection, i.e., protection against a failure in one of the primary links 102 or 112, it fails to provide span protection, where span protection refers generally to an ability to switch locally from a primary trunk to a backup trunk. In addition, since a full unit of the OC-x traffic is assigned to each link, this approach does not accommodate preemptible traffic, and fails to provide any opportunity for quality of service (QoS) enhancement.
More sophisticated approaches may involve the use of a path restoration algorithm to provide automatic restoration of network traffic in the event of a primary path failure, while sharing restoration capacities whenever possible, so as to reduce the total amount of required redundant capacity.
Examples of known path restoration algorithms are described in, e.g., U.S. Pat. No. 6,021,113 issued Feb. 1, 2000 in the name of inventors Bharat T. Doshi et al. and entitled “Distributed Precomputation of Network Signal Paths with Table-Based Link Capacity Control,” J. Anderson, B. T. Doshi, S. Dravida and P. Harshavardhana, “Fast Restoration of ATM Networks,” JSAC 1991; W. D. Grover, “The Self-Healing Network: A Fast Distributed Restoration Technique for Networks Using Digital Cross Connect Machines,” IEEE Globecom 1987; U.S. Pat. No. 4,956,835, issued to W. D. Grover on Sep. 11, 1990; C. H. Yang et al., “FITNESS: Failure Immunization Technology for Network Service Survivability,” IEEE Globecom 1988; C. Edward Chow, J. Bicknell, S. McCaughey and S. Syed, “A Fast Distributed Network Restoration Algorithm,” IEEE Globecom '93, pp. 261–267, 1993; and S. Hasegawa, Y. Okanone, T. Egawa and H. Sakauchi, “Control Algorithms of SONET Integrated Self-Healing Networks;” and U.S. Pat. Nos. 5,435,003 and 5,537,532, both entitled “Restoration in Communications Networks” and issued to R. S. K. Chng, C. P. Botham and M. C. Sinclair.
These more sophisticated approaches are often computationally intensive, and are therefore not appropriate or desirable in many applications. What is needed is a simple approach which utilizes redundancy but also overcomes the above-noted problems associated with the conventional “bridge and select” approach.